The present invention relates to a toner image developing device for developing a latent image formed on an image carrying body with a color toner.
An image-forming device such as a copier or a printer employing electrophotographic method is usually provided with a conventional developing device for developing a latent image formed on a light-sensitive surface of an image carrier with color toner transferred thereto to visualize the image.
In the conventional developing device, a latent image formed on the light-sensitive drum and developed with toner thereon is transferred onto a sheet of printing material. After this, toner particles remaining on the surface of the light-sensitive drum are removed off before starting a subsequent process of forming a latent image thereon. For this purpose, a cleaning device is provided for cleaning off unused toner remaining on the light-sensitive drum after transferring the developed toner-image therefrom onto the printing material. The removed toner particle is collected in a toner collecting portion of the cleaning device.
The developing device is provided with a developer roller for transporting a developing agent composed of toner particles and magnetic carrier particles to a developing area opposing to the light-sensitive drum by using magnetic force. A developing agent remaining unused after developing a latent image is returned into the developing chamber. The developer roller is supplied with fresh developing agent instead of a collected agent to obtain high-quality of toner-developed images. This stabilizes the latent image developing process so that a sufficient visual image can be always obtained.
A developing agent remaining on the developer roller after developing a latent image is separated by using a magnet disposed in a non-magnetic cylindrical sleeve. This magnet has an odd number of magnetic poles, neighbors of which are of the same polarity at an area for removing a developing agent from the developer roller after developing a latent image. This realizes that a developing agent can naturally fall down from the developer roller.
In the above-mentioned developing device, the developer roller is disposed opposite to the light-sensitive drum to visualize a latent electrostatic image formed the light-sensitive drum. FIG. 1 schematically shows an exemplified construction of the developing device. As seen in FIG. 1, a developing chamber 101 containing a developing agent composed of toner particles and carrier particles is provided with a developer roller 102 rotatably mounted therein at an opening portion opposite to a light-sensitive drum (not shown). The developer roller 102 is partly sticking out through the opening of the developing chamber 101 and is disposed at a specified space (gap) from the light-sensitive drum (not shown).
The developer roller 102 consists of a rotary cylindrical sleeve 102a made of non-magnetic material containing a magnet roller 102b having the odd-number of magnetic poles. The magnetic roller 102 is disposed so that its main developing magnetic pole N1 is opposite at a specified gap to the light-sensitive body.
No adsorption of the developing agent occurs between magnetic poles S2 and S3 of the magnet roller 102b. Therefore, developing agent falls off the corresponding area surface of the sleeve 102a. A stirrer roller 103 is disposed opposite to the above-mentioned area of the sleeve 102a. This stirrer roller 103 is provided for mixing a developing agent and transporting the mixed agent to the position of a suction magnetic pole S3 of the magnet roller 102b. The developing agent is adsorbed to the surface of the sleeve 102a and further transported by rotation of the sleeve 102a.
Thus, the sleeve 102a with a developing agent adhered to its surface by the magnetic force of the magnet roller disposed therein rotates in the direction indicated by an arrow in FIG. 1. Namely, the sleeve 102a rotates to transport a developing agent sucked by the magnetic pole S3 onto the sleeve surface and further adhered thereto by the attracting force of the so-called transporting magnetic poles N2 and S1 of the magnet roller 102b. A doctor plate 104 is disposed opposite to the rotating sleeve 102 within the area between the magnetic poles N2 and S1 therein to restrict thickness of developing agent layer adhering to the rotating sleeve. The developing agent is further transported to an area of the developing magnetic pole N1 and is magnetically excited (like a magnetic brush) to be in contact with the surface of rotating light-sensitive drum. The latent image formed thereon is thus developed.
At the upstream side of the developing roller, there are disposed two screw rollers 105 and 106 for feeding toner particles in the direction of its rotation axis to a stirrer roller 103. These screw rollers 105 and 106 are arranged in parallel to the developer roller 102 and the stirrer roller 103 in horizontal direction.
As shown in FIG. 2, the toner feeding screw rollers 105 and 106 have a parallel rotation axis and are driven into rotation in reverse directions to each other. Toner is fed from a supply inlet 107 (as indicated by an arrow P) to one end of the screw roller 105 by which the particles are transported in the direction shown by an arrow (leftward in FIG. 2) and transferred to the screw roller 106. The particles are then fed by the screw roller 106 to the stirrer roller 103 disposed in parallel to the screw roller 106.
The thus constructed developing device is disclosed for example in Japanese Laid-Open Patent Publication No. 2-64583. The developing device of this type is very small to be adapted for manufacturing a thin copier or printer. This is particularly adapted to dispose a plurality of developing devices containing toners of different colors (e.g., yellow, magenta, cyan and black) respectively in layers in respect to a light-sensitive drum.
The developing device shown in FIGS. 1 and 2 has an advantageous thin design achieved by horizontal arrangement of the stirrer roller 103 for stirring a developing agent, the developer roller 102 and the transporting screw rollers 105 and 106 for feeding toner in a tank.
In recent years, a rapid advance has been made in digital and multicolor image forming apparatuses such as copiers, printers and facsimiles. In particular, a variety of image forming methods have been proposed for color image-forming apparatuses.
A typical arrangement is such that four developing devices as described before are disposed one above another around a light-sensitive drum to repeat four developing cycles for forming a color image. For this purpose, the arrangement of four developing devices of FIG. 1 may effectively prevent the image-forming apparatus being increased in height. However, the developing process must be separately performed four times, resulting in reduction of copying speed of the apparatus.
Such a tandem color-image forming method has been proposed with four light-sensitive drums which are arranged parallel to one another, and four different color images which are formed thereon, developed with respective color toners and transferred onto an intermediate transfer drum or directly onto a copy sheet. This method can obtain four-color toner-developed images at the same time, thereby achieving four-times higher copying speed as compared with the before-described method. In this regard, the tandem method is very advantageous color-image forming method.
The tandem method of forming a color image requires parallel arrangement of light-sensitive drums for forming respective color images and respective processing means including developing devices. If the developing devices of FIG. 1 are provided in parallel for respective light-sensitive drums, they have an increased size in the horizontal direction. In the tandem system in particular, it is desired to increase the copying speed by shortening the distance of transporting a recording sheet, which may be realized by arranging the light-sensitive drums at a least interval between them. This requirement cannot be satisfied by the developing devices of FIG. 1.
The developing device of FIG. 1 is provided with only one stirrer roller 103 which cannot achieve high efficiency of mixing toner particles with a developing agent and, therefore, cannot be adapted to a high-speed developing process. The use of plural stirrer rollers 103, however, increases the horizontal length (i.e., the width) of the developing device.